Analytical and Experimental Study of Residual Stresses in CFRP

Fiber Bragg Grating sensors (FBGs) have been utilized in various engineering and photoelectric fields because of their good environment tolerance. In this research, residual stresses of carbon fiber reinforced polymer composites (CFRP) were studied using both experimental and analytical approach. The FBGs were embedded inside middle layers of CFRP to study the formation of residual stress during curing process. Finite element analysis was performed using ABAQUS software to simulate the CFRP curing process. Both experimental and simulation results showed that the residual stress appeared during cooling process and the residual stresses could be released when the CFRP was machined to a different shape. 1. Introduction Carbon fiber composites have been widely considered as the optimal replacement material for various industrial products. For composite materials, the inherent defects could greatly hamper the reliability and durability of the resultant products. There have been many studies of embedded stress sensors, particularly for damage detection in composite materials [1, 2]. FBGs possess great compatibility with CFRP [3], and by embedding FBG inside carbon fiber composites, the residual strain of the carbon fiber composites during production could be easily monitored. Because of thermal expansion, coefficient of the elements in the composite materials is different, and the shape of the optical spectra changed during curing process. These changes induced a shift in wavelengths of the optical fiber sensors, which could be converted to residual stress [4]. In this research, residual stresses of carbon fiber reinforced polymer composites (CFRP) were studied using both experimental and analytical approach. The simulation of residual stress during cooling process was carried out using ABAQUS. Through comparison between experimental and simulation results, the formation of residual stress in the CFRP was confirmed. 2. Materials and Methods 2.1. Fiber Bragg Grating Sensors (FBG) The FBG involved was fabricated from single cladding photosensitive fiber using the side writing method. The photosensitive fiber was produced by Fibercore Co. Ltd. (PS1250/1550). The FBGs are photoimprinted in photosensitive optical fiber by 248？nm UV radiation from a KrF Excimer laser. The impulse frequency of laser is 10？Hz. Along the fiber core, the FBG has periodic refractive index modulation with a period of 1.05~1.08？μm. This resulted in a peak Bragg reflecting wavelength of 1540~1564？nm. The reflectivity of the resulting FBG was about 99% and the FWHM (Full width Half Maximum)